This video explains how to check the ground insulation of an AC motor winding using the insulation resistance (IR) test.

We often hear the terms preventive maintenance (PM) and predictive maintenance (PdM) of electric motors. However, it’s far less often that we give consideration to the tasks associated with these methods of maintaining motor operation and extending operating service life. This article will address some of the more common activities associated with PM and PdM, with the focus on three-phase squirrel cage motors.

There is much discussion in the industry about how to properly electrically test AC stator and wound rotor windings. Topics include test voltage, procedure, frequency and when to perform the various tests. This article describes how the following standards address these questions:

• NEMA MG 1-2011
• (MG1) IEEE 43-2000
• (IEEE 43) IEEE 62.2-2004
• (IEEE 62.2) IEEE 522-2004
• (IEEE 522) IEEE 1068-2009
• (IEEE 1068) ANSI/EASA AR100-2010
• (AR100) CSA C392-2011 (C392)

These standards are regularly reviewed and coordinated, so some of the information may not match the old yellowed reference taped to your toolbox lid. These updated references should replace anything dated previous to the dates indicated on the standard. AR100 Section 4.3.1 lists the recommended tests for stator and wound rotor windings. They are insulation resistance (IR), winding resistance, growler, phase balance, surge comparison, polarity and ball rotation tests. This article covers the IR, winding resistance and surge tests. Noticeably absent from this list is the ever popular high potential (hi-pot) test. Topics covered also include: IR (or megohm) test Polarization index test Winding resistance test Surge comparison test Hi-pot test.

All of us in the electrical apparatus service industry test the winding ground insulation resistance of machines such as motors and generators. A frequent question is: What is the minimum acceptable megohm value for this winding? The good news is that there is a standard that identifies minimum values for insulation resistance of rotating machines. That standard is the "IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery," IEEE Std 43-2000. The EASA "Recommended Practice For The Repair Of Rotating Electrical Apparatus," ANSI/EASA AR 100-2010, uses IEEE 43 for its insulation resistance test references. Note that IEEE 43 only applies to rotating machinery. There is no equivalent standard for non-rotating electrical machinery such as transformers. In this article we will delve into determining minimum insulation resistance for rotating electrical machinery.